4th International Conference on Sustainable Energy and Environmental Engineering (ICSEEE 2015)
Removal of elemental mercury and divalent mercury with ammonium
sulfide in smelting gas containing SO2
Xueqian Wang1, a,*, Jianlin Xiong2,b , Langlang Wang3,c and Shudi Yan4,d
1, 2, 3and 4
a
Faculty of Environmental Science and Engineering, Kunming University of Science and
Technology, Kunming 650500, Yunnan, China.
[email protected], [email protected], [email protected], [email protected]
Keywords: (NH4)2S, simultaneous removal, Hg0, Hg2+, SO2
Abstract. A set of simulation equipment is set up after the conditions of removing SO2 and heavy
metal Hg (Hg0 and Hg2+) has been investigated, the results of the removal technology method of
absorption Hg0, Hg2+ in smelting gases containing SO2 with (NH4)2S is tested, and their
composition of reaction products and sediment are analyzed with XRD and XPS. Obtained that:
when absorption concentration of (NH4)2S are 0.8 mol/L, in pH 7.0-3.0 removal efficiency of Hg0
and SO2 is higher, temperature will affect the absorption of Hg0, when reaction temperature is 30˚C
the absorption of Hg0 can be achieved the best. Under these optimum conditions, the removal
efficiency of Hg0, Hg2+ and SO2 could reach 89.45%, 99.42% and over 99.99%, the participation of
SO2 has a promote consequence on removal of Hg0. The main reaction product is HgS, HgSO4,
(NH4)2S2O3.
Introduction
Non-ferrous smelting process would produce a large amount of flue gas, which main composition
of smelting fumes are SO2, SO3, H2O and heavy metals. Among them, Mercury is one of the most
toxic heavy metals. In China, zinc smelting atmospheric mercury emissions reach to 49.1t, mercury
emissions of Hg0, Hg2+ and Hgp is 78%, 17% and 5% of total mercury emissions respectively recent
years. Hg0 has gradually become a leading concern due to its high toxicity, it possesses high
volatility and low solubility in water, and it is easily spread for a long time and long distances in the
atmosphere. [1] The concentration of SO2 is about 0.05% to 25% and SO2 emissions constitute more
than 8 percent of China's total SO2 emissions [2].
For processing heavy metals from the flue gases, most research in internal and abroad, mainly
by adsorption [3,4,5,6] and catalytic oxidation method [7,8,9]. But most of them either have a high
economical cost or some of them have the problem of secondary pollution. Thus the developing of
low costs and low pollution technology for mercury removal is still necessary. For the treatment of
SO2 smelting gas, there are related research in internal and abroad. This experiment chooses
(NH4)2S as absorbing liquid for the removal of SO2, and it also has a relatively high absorption rate
for Hg2+ and Hg0. This paper focuses on the study of removal of bivalent mercury (Hg2+), elemental
mercury (Hg0) and in simulated flue gas containing SO2 by (NH4)2S solution.
Experimental
Materials and methods
The experimental device for using ammonium sulfide to remove Hg0, Hg2+ and SO2 in simulated
© 2016. The authors - Published by Atlantis Press
416
flue gas is shown in Fig.1. In this experimental, Hg0, Hg2+ simulated flue gas was the mixed with
SO2 of single Hg0 and Hg2+. The corresponding tail gas absorption solution of Hg was acidic
solution of KMnO4, and the corresponding tail gas absorption solution of SO2 was 5% NaOH.
Because Hg0 is easy to volatilize, u-shaped tube is used for generation of Hg0. Dried compressed air
is used to access into mercury vapor tube (Accounting for the total amount of oxygen is about 21%),
flow rate is 10~20mL/min, inlet the mix bottle to mixed with SO2. And for Hg2+, put the soiled-state
mercuric chloride (HgCl2) into a tubular resistance furnace (SLG1200-60, 220V, 4000W), then
heated tubular resistance furnace to 400ºC, then carrier gases were the mixture of nitrogen and
sulfur dioxide with a constant flow rate of 200 mL/min. inlet concentration of Hg0 is maintained at
20~40mg/m3, inlet concentration of Hg2+ is 10~30 mg/m3, which is correspond concentration of Hg0
in smelting gases.
Fig.1 Simultaneously removal experimental device of absorbing smelting gas Hg0, Hg2+ and SO2
with (NH4)2S.
The experimental principle
The principle of Hg2+ removal can be fully explained by the equation ((Eq.1), Mercury ions (Hg2+)
reacted with sulfide ions (S2-) to generate stable mercuric sulfide precipitate under the condition of
weak alkaline, [10]. And as shown in (Eq.2-3), because of the existence of SO2 in simulated flue gas
react with the solution, and the reaction product between sulfur dioxide and ammonium sulfide,
hydrogen sulfide (H2S) and sulfur (S), can react with mercury (Hg0), the principle of Hg0 removal
was shown in ((Eq.4-5).
Principle of (NH4)2S absorb SO2: According produces H2S and no H2S, the absorption process
into pH> 7 stages and pH <Stage 7. And the overall reaction chemical equation is shown in Eq.2-3.
Hg 2  S 2  HgS 
(1)
6SO2  4( NH 4 ) 2 S  4 H 2 O  O2  2( NH 4 ) 2 S 2 O3  4 NH 4 HSO4  2 H 2 S
(2)
( NH 4 ) 2 S 2 O3  2( NH 4 ) HSO3  2S  2( NH 4 ) SO4  2 H 2 O
(3)
Hg 0  S  HgS 
(4)
Hg 0  H 2 S  HgS   H 2
(5)
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Results and discussion
Effect of (NH4)2S absorption concentration on removal efficiency
Adding (NH4)2S 100mL which concentration is 0, 0.2, 0.4, 0.8 and 1.0 mol/L respectively in
absorption bottles, removal efficiency of Hg0, Hg2+ and SO2 are affected with concentration of
(NH4)2S, the test results are shown in Fig.2. As can be seen from the Fig.2, when the concentration
of (NH4)2S solution was increased from 0 to 0.2 mol /L, the removal efficiency of Hg0 also
increased from 50.89% to 67.40%, removal efficiency of Hg2+ increased from 82.64% to 95.06%
respectively, and the removal efficiency of SO2 also increased from 88.7% to 98.8%. When the
concentration of (NH4)2S solution increased from 0.2 to 1.0mol, the removal efficiencies of Hg0,
Hg2+ and SO2 could reach 87.71%, 99.96% and 99.92% respectively. As can be seen from equations
(1-5), when the concentration of (NH4)2S solution increased the concentration of S2-, S and H2S
increased too.
1.0
Removal Efficiency
0.9
0.8
0.7
0
0.6
Hg
2+
Hg
SO2
0.5
0.0
0.2
0.4
0.6
0.8
1.0
(NH4)2S Concentration(mol/L)
Fig.2 The influence of concentration on removal of mercury and SO2 from simulated flue gas.
(Concentration of SO2 was 6%)
Effect of SO2 absorption concentration on removal efficiency
Adjusting concentration of SO2 gas to 0%, 2%, 4%, 6%, 10% respectively, removal efficiency
of Hg0, Hg2+ and SO2 are affected with concentration of (NH4)2S, the test results are shown in Fig.3.
As the concentration of SO2 from simulated flue gas was increased from 0% to 2%, the removal
efficiencies of Hg2+, SO2, decreased from 99.64%, 100%, to 99.05%, 99.99%, but the removal
efficiencies of Hg0 increased from 76.46% to 80.61% (Fig.3). When the concentrations of SO2 from
2% to 10%, the removal efficiencies of Hg0, Hg2+, SO2, decreased from 80.61%, 99.05% and
99.99% to 71.08%, 98.44% and 99.57%.The participation of SO2 leads to pH of the solution is
reduced, an acidic conditions more conducive to the absorption of Hg0, and SO2 react with (NH4)2S
solution produces S and H2S can react with Hg0. But as the reaction between SO2 and (NH4)2S
proceeds are consuming solute, which makes removal efficiency of Hg0, Hg2+ decline.
418
1.00
Removal Efficiency
0.95
0.90
0
Hg
2+
Hg
SO2
0.85
0.80
0.75
0.70
0
2
4
6
8
10
SO2 Concentration(%)
Fig.3 The influence of SO2 on removal of mercury and SO2 from simulated flue gas.
(concentration of (NH4)2S was 0.8mol/L, temperature of (NH4)2S was 20˚C)
The influence of pH value on removal efficiency
Continue experiment to be done in different pH, and averaging pH is used to observe the
changes of pH value in absorption process, results are shown in Fig.4. As can be seen from Fig.4,
when pH is from 12 to 7, absorption rate of Hg0 increases from 48.89% to 74.67% with pH decrease.
Acidic base of pH <7.0 is more beneficial to Hg0’s absorption, but the range of removal efficiencies
for Hg2+ decrease, were 99.16%-97.14%, when the pH value was nearby 7, the removal efficiencies
reduced to a minimum level. When pH is higher than 4.0 removal efficiency of SO2 remains over
99.9%, pH has little effect on the absorption of SO2. Gaseous Hg0 can be dissolved into ions easier
when the solution was acidic base, but when pH is reduced to below 3.0, absorption rate is low to
54.95% because of the (NH4)2S solute consumption. But no matter under acidic or alkaline base,
ions state of mercury (Hg2+) can easily reacted with sulfur ions.
1.0
Removal Efficiency
0.9
0
Hg
2+
Hg
SO2
0.8
0.7
0.6
0.5
12
10
8
6
4
2
pH
Fig.4 The influence of pH value on removal of mercury and SO2 from simulated flue gas.
(Concentration of (NH4)2S was 0.8mol/L, concentration of S O2 was 6%, temperature of (NH4)2S
was 20˚C)
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The influence of temperature on removal efficiency
Place (NH4)2S 100mL which concentration is 0.8mol/L in absorption bottles of series two stage
absorption, continue experiment in different reaction temperature (20˚C,30˚C, 40˚C, 50˚C, 70˚C)
respectively, results are shown in Fig.5. As can be seen from Fig.5, when the temperature increased
from 20˚C to 30˚C, removal efficiencies of Hg0 and Hg2+ both increased, reach to 89.45% and
99.42% when the temperature was increased from 30˚C to 70˚C, the removal efficiencies of Hg2+
were not significantly increased, and the removal efficiencies of Hg0 turn to decreased to 71.43%,
the removal efficiencies of Hg2+, SO2 were not significantly increased. Due to a range of rise
temperature due to the accelerated the molecular motion to some extent, but the removal efficiency
of ascension, but for Hg0, the rise of reaction temperature will promote its volatile, leading to
removal efficiency decreased.
1.00
0
Hg
2+
Hg
SO2
Removal Efficiency
0.95
0.90
0.85
0.80
0.75
0.70
20℃
30℃
40℃
50℃
70℃
(NH4)2S Temperature(℃)
Fig.5 The influence of temperature on removal of mercury and SO2 from simulated flue gas.
(Concentration of (NH4)2S was 0.8mol/L, concentration of SO2 was 6%, initial pH value of (NH4)2S
was 8)
Characterization of materials
XRD analysis
As shown in Fig.6 HgS characteristic peak is obvious that the reaction substance is the
mercuric sulfide and have complete crystal shape, which proved that when solution absorb Hg0 in
the presence of SO2 a small amount of black precipitates appears, and its composition is mainly
HgS, it is same as Hg2+.
16000
◆
14000
◆HgS
12000
Intensity
10000
8000
6000
◆
4000
◆
◆
2000
◆
0
-2000
0
20
40
60
80
100
2Θ(°)
Fig.6 XRD patterns of (NH4)2S removal Hg0 generated precipitation.
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XPS analysis
In order to further determine the products of reaction，XPS analysis was used to measure the kind
and composition of the absorption liquid crystal, Analysis results show that in Fig.7. major S2p
peaks centers at 168.78eV and 168.42eV, verify that the main form of products in solution is SO42and S2O32-.And the S2p 3/2 peak center 170.45 indicate the presence of H2S. [11]
2-
SO4
2-
CPS
SO4 2H2O
H2S
172
2-
S2O3
170
168
166
Binding Energy(eV)
Fig.7 XPS patterns of absorbing liquid crystalline of (NH4)2S removal of SO2.
Conclusion
Considering removal efficiency and economic costs, to removal of Hg0 and Hg2+ from metallurgical
off-gas contains the SO2 with (NH4)2S as the absorption liquid, the optimal concentration, pH value
and temperature of (NH4)2S solution were 0.8 mol/L, 6 and 30˚C respectively. Under these
conditions, the removal efficiencies of Hg0, Hg2+ and SO2 from simulated flue gas could reach
89.45%, 99.42%, 99.99%, respectively. The results of XRD and XPS shows that the mechanism of
Hg’s removal was mainly chemical deposition, and the participation of SO2 has a promote
consequence on removal of Hg0. The main products is likely to be HgS, (NH4)2S2O3, (NH4)2SO4,
and so on, it’s ease of secondary use.
Acknowledgments
This work was supported by the National Natural Science Foundation of China under Grant No.
51268021, No. 51368026, U1137603.
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